Being capable of receiving weak far-infrared rays emitted from bodies for the purpose of detecting the existence of bodies in a certain area, pyroelectric infrared body sensing switch circuits are widely used in body detection applications such as burglar alarms, equipment safeguard devices, lighting controls in public places of dwelling houses, etc. Pyroelectric infrared body sensing switches are especially useful for lighting in public places of dwelling houses, where the switch can turn on automatically when a person passes by and keep a light on for a certain delay time, and then turn the light out after the person leaves. In this way, the switch can save much energy. The switch conforms to the environmental protection concept and is convenient for use.
Therefore, pyroelectric infrared body sensing switches are used for lighting control in more and more public places of dwelling houses. The present pyroelectric infrared body sensing switch circuit normally consists of a power supply unit, infrared detection & signal amplification unit, delay unit and execution unit. After an infrared signal from a human body is detected by the infrared detection device and is amplified by the signal amplification unit, it triggers the delay unit to start the time delay. After the preset time lapses, it outputs a signal to the execution unit to turn off the light or to control other appliances to act accordingly. Normally the delay unit adopts the time constant of the RC charge circuit, which is composed of a charge capacitor and a charge resistor as the reference. As restricted by the capacitance and volume of the capacitor, this mode can not provide a long time delay. Meanwhile, when the capacitance of the capacitor is more than 200 μF, the timing accuracy deteriorates, causing certain restrictions to the application occasions of the present pyroelectric infrared body sensing switch.
In addition, the power supply unit of the present pyroelectric infrared body sensing switch normally adopts a single power supply to provide the whole switch with power. There are two commonly used circuit forms. One is an RC voltage reduction mode, which has the advantages of low cost and small volume but also has the disadvantage of a relatively low power of the power supply. The other mode is a transformer voltage reduction mode. Although this mode can work more reliably and can provide a higher power, the volume of the power supply is large and the cost is high.